This invention relates to anaerobic adhesive compositions, reaction products of which demonstrate enhanced cure speed and cure strength as compared to conventional anaerobic compositions. More particularly, the compositions of the present invention reach a high percentage of their final cure strength at room temperature cure in very short time periods, e.g., in about one hour or less.
Structural anaerobic adhesives are used in a wide variety of applications. Typically, curable monomers used in these applications have been tailored to provide the strength and toughness required for the application at hand. In addition to adequate cure strength and toughness properties, resistance to chemicals and temperature extremes, and the ability to cure rapidly are also properties often desired. Structural anaerobic adhesives are used for a number of important applications in the automotive industry, where production line efficiencies are important.
Adhesive bonding strength, which is among the most important properties of structural adhesives, is often difficult to achieve on coated substrate surfaces frequently encountered with automotive parts. Among the surfaces on which it is particularly difficult to obtain rapid, high strength bonding are those which have been coated with cured adhesive, paint and various other finishes or surface preparations common to the automotive industry. These coatings often times prevent direct contact between the substrate and the applied adhesive, thereby interfering with the bond intended to be formed therebetween. For example, coating of paints and epoxies on metal parts is common in the automotive industry. In one particular application, motor housings are electrostatically coated with epoxy resins in the production process. Motor magnets are then bonded to the cured epoxy coating using structural anaerobic adhesives, which are then expected to demonstrate sufficient bond strength to withstand the useful life of the motor. Current anaerobic structural adhesives require about 24 hours or longer to reach significant cure strength. This time delay is costly from a production standpoint. Additionally, the final cure strengths of most commercial anaerobic structural adhesives, while adequate for many applications, leave room for improvement both in final cure strength and, as noted above, in the speed in which cure strength is reached.
Those persons of ordinary skill in the art acknowledge that peroxides serve as a free radical generating source which initiate free radical curing of the polymerizable anaerobic adhesive monomer compositions. To increase the speed at which the free radical is generated, accelerators are employed in combination with the peroxides to enhance the speed at which the peroxide free radical is generated. In so doing, the cure speed of the anaerobic adhesive composition is increased. As anaerobic adhesive compositions have evolved, various combinations of peroxides and accelerators have been used. Recently, such compositions have employed a plurality of compounds as accelerators to serve as reducing agents for the peroxide to increase cure speed.
One approach to achieving accelerated cure speeds is to add accelerators to the anaerobic adhesive compositions.
Various accelerator compounds, such as tertiary amines, imides, polyamines, cyclicamines and arylamines, have been successfully included in anaerobic adhesive compositions to further increase cure speeds and completion of cure. See, e.g., U.S. Pat. No. 3,041,322 (tertiaryamines), U.S. Pat. No. 3,046,282 (imides), U.S. Pat. No. 3,203,941 (polyamines), and U.S. Pat. No. 3,218,305 (cyclic and arylamines), all to Kriebel. These accelerators were generally used in amounts of about 0.05 up to about 20%, depending on the type of amine and amount of peroxide present in the anaerobic adhesive composition. While the use of accelerators tends to increase cure speed, it is believed to have little effect on increasing the ultimate bond strength.
U.S. Pat. Nos. 4,287,330 and 4,321,349 to Rich disclose anaerobic adhesive compositions which include a hydrazine accelerator, along with peroxides and amines in amounts previously referred to herein, to increase the cure speed of the anaerobic curing monomer. The hydrazines are disclosed as being useful in amounts of about 0.1-5% by weight.
Anaerobic adhesive compositions have also conventionally been used with primer compositions to speed their cure. On substrates such as stainless steel, zinc, dichromate, and cadmium, which are considered xe2x80x9cslowxe2x80x9d or relatively inactive materials for anaerobic cure, primer compositions have been considered necessary for quick fixture and cure times required of many applications.
The use of primer compositions requires an additional step prior to applying the anaerobic adhesive composition, which is often inconvenient and costly. Often the solvent used to carry the accelerator component in the primer is environmentally harmful and requires special handling and disposal. Moreover, ordinarily the user must wait until the solvent has evaporated before applying the adhesive.
Adhesion promoters have also been added to anaerobic adhesive compositions to improve adhesion. Typically, silanes have been used to modify the surface chemistry of filler materials. Cure speed is not usually affected by the use of silanes and increased bond strength, as opposed to better adhesion, does not always occur. Silanes often require heat to react with fillers and have handling problems due to rapid hydrolysis and polymerization in the presence of moisture.
U.S. Pat. No. 4,539,049 discloses aluminum zirconium metallo-organic complexes which are used as coupling agents. These complexes are used to modify the surfaces of particulates and fibers which are then added to resin materials to enhance physical properties. No express disclosure of specific resin materials is provided, nor is bond strength or acceleration of cure mentioned.
U.S. Pat. No. 5,656,703 to Costin et al. discloses using a metal di(meth)acrylate and a polyamine in a curable coating composition to obtain improved adhesion. No mention of cure speed, acceleration of completeness of cure or higher bond strength is made.
U.S. Pat. No. 4,916,184 to Clark discloses the use of specific initiator compounds, such as t-butyl peroxy maleic acid to achieve increased bond strengths of ethylenically unsaturated adhesive compositions. No mention of cure speed or rapid completeness of cure is made.
Chartwell International, Inc., Littleborough, Mass., markets a series of adhesion promoters under the trademark CHARTWELL. One particular adhesion promoter sold by this company, Chartwell B-545.1, is described by their publicly available data sheet (5/00) as being useful in conjunction with xe2x80x9cacrylate (meth)acrylate adhesives and sealants for improved adhesion to metals, concrete, wood and many plastics (oxygen containing polymers only) and ceramics. No disclosure or suggestion as to speed of cure, obtaining increased bond strength or reaching a greater percent of ultimate bond strength in the first hour is made.
Thus, although a variety of accelerator compositions and adhesion promoters have been employed to increase the cure speed of anaerobic adhesive compositions, achieving ultimate bond strengths in rapid time periods employing such accelerators have not been entirely successful.
It would be therefore desirable, to provide anaerobic compositions which provide high bond strength in an extremely rapid fashion, which readily reach a high percent of their ultimate bond strength and which bond well to surfaces, such as finished metal surfaces and plastics, to which bonding is traditionally difficult. The present invention provides compositions which achieve these and other advantages to be further disclosed herein.